Controlling a polarization mode and a spatial mode of optical signals in photonic integrated circuits (PICs) is important for a fiber optical communication network. For example, a conventional single mode fiber that can be used in the network does not preserve the polarization mode. When the optical signal is coupled from the single mode fiber to the PICs, the signal decomposes into arbitrary compositions of two orthogonal polarization components, namely, a first component in a transverse electric (TE) mode and a second component in a transverse magnetic (TM) mode. In many modules used in the PICs, the components in the TE and TM modes exhibit different characteristics. For example, the components having different TE and TM modes propagate at different velocities in a high index contrast waveguide, and the energy coupling coefficients of a microring resonator for the TE and TM modes are different.
These polarization-dependent effects degrade the performances of the PICs, especially for high speed communication. Also, most optical communication networks use only one polarization mode. Furthermore, if the components in both polarization modes are used in polarization-division multiplexing (PDM) systems, the spectral efficiency of such systems can be increased.
In addition to the polarization modes, the optical signal and/or each component of the optical signal can have various spatial mode orders. For example, a signal in a fundamental mode order has a lowest propagation loss in the waveguide.
Typically, systems for controlling polarization of optical signals, e.g., polarization transparent systems and polarization multiplexing systems, use polarization converters and/or polarization splitters. The systems for controlling spatial order mode include a multi-mode interference coupler, a directional coupler, and a Y-shaped coupler. In general, the converters can be categorized as two types, i.e., mode-coupling converters and mode-evolution converters.
The mode-coupling converters are typically composed of two waveguides with different geometries connected via an abrupt junction transition. The mode-coupling uses beating behaviors between a pair of waveguide modes in the second waveguide excited by a specific mode in the first waveguide. The beating is due to the combination of two slightly unequal frequencies produces a “beat”, resulting from the tones cycling in and out of phase with each other. The mode beating behavior is determined by geometry of the device and operating wavelength of the signal. Therefore, the mode-coupling converters are inherently sensitive to variations in the fabrication process, and wavelength dependent.
The mode-evolution converters replace the abrupt transition with gradual variation of the waveguide geometries along the wave propagation direction. Along the converter, a mode in the first waveguide can evolve into another mode in the second waveguide with different polarization and spatial distribution without exciting other modes in the second waveguide. Compared with mode-coupling converters, the mode-evolution converter has a longer length, a larger bandwidth and a better tolerance to fabrication variations.
Unfortunately, it is not always possible to achieve polarization control and spatial mode control with one type of converter. For example, a mode-evolution polarization converter for transforming a signal in the fundamental TM mode (TM0) into the signal in the fundamental TE mode (TE0) is complicated and requires, e.g., asymmetric bi-level tapers to be used to achieve geometry asymmetry in both horizontal and vertical direction. Such converter has a small tolerance to fabrication variations. Therefore, different converters, which can control the polarization and spatial mode order individually, are often combined into compound converter. However, the compound converter also has a number of fabrication and configuration problems. For example, a polarization converter and a spatial converter may have different epitaxial-grown or etched structures, which are difficult to realize in a simple fabrication process.
Accordingly, conventional solution includes combination of mode-evolution and mode-coupling converters. For example, one conventional converter includes a deep etched width taper, i.e., in the mode-evolution converter, connected to directional coupler, i.e., the mode-coupling converter. Another converter includes a constant width waveguide, i.e., a mode-coupling converter connected to asymmetric Y-coupler, i.e., mode-evolution converter. As an advantage, such converters can have a relatively small length. However, a mode-evolution device in series with a mode-coupling device does not preserve the benefits of using the mode-evolution device, such as the larger bandwidth and the fabrication tolerance, though using mode-coupling based converter may reduce the total device length.
Accordingly, there is a need to design an optical converter that has a large bandwidth and is simple in fabrication.